The invention relates to a double-layer fabric for the sheetforming section of a papermaking machine which is a so-called sheet-forming fabric or a dewatering fabric. A double-layer fabric is understood to be a woven fabric in which the transverse threads are disposed in a lower and an upper layer and are generally located one above the other pairwise. The transverse threads are interwoven with a single system of longitudinal threads.
In the formation of the paper sheet in a papermaking machine, the water is removed from the aqueous fiber suspension by means of the dewatering fabric until fiber web is formed on the dewatering fabric which is sufficiently strong to be removed from the forming fabric and to be introduced into the press section.
The dewatering fabric must satisfy many varying requirements, namely a high dewatering efficiency, a fine and planar surface structure on the paper-carrying side, a good fiber retention, a high longitudinal and transverse stability and a high abrasion resistance.
In addition the dewatering fabric must exhibit good running stability, i.e., it may not become distorted and must exhibit perfect straight-run characteristics at speeds of from 1000 to 1500 m/min. and may not drift or run off to the side.
The problem of drifting or running-off of the dewatering fabric toward the side of the machine is not encountered with all types of weaves. It depends primarily upon the symmetry of the transverse thread floats on the running side. In particular, the running side of double-layer forming fabric is formed by transverse thread floats in order to improve the resistance to abrasion and the service life.
A fabric for the sheet-forming section of a papermaking machine is disclosed in U.S. Pat. No. 4,709,732 granted Dec. 1, 1987, This dewatering fabric is a double-layer fabric and the transverse thread floats on the running side are asymmetrical with the lowest point of the transverse thread floats being shifted from the center toward one side. The asymmetrical transverse thread floats lead to an asymmetrical bearing surface of the dewatering fabric with the result that at high speeds the dewatering fabric drifts toward the side. The lateral drifting is highest when a vacuum is applied on the suction boxes for removing residual water from the fiber web. The force by which the dewatering fabric is urged downwards against the papermaking machine is increased through the vacuum whereby the asymmetry of the transverse thread floats has a greater effect. The fabric guide roll must then be set obliquely in order to retain the dewatering fabric in the papermaking machine. If that does not suffice, additional rollers in the papermaking machine must be set obliquely with transversely directed forces resulting therefrom which counteract the lateral drift of the dewatering fabric. If the vacuum of the suction boxes is, for operational reasons of the machine reduced or cut off, the dewatering fabric will escape in the opposite direction by the action of the rollers which are still set obliquely whereby the fabric is frequently damaged on account of the impact as it hits against the framing of the papermaking machine. These difficulties arise especially during the first few days of work with the dewatering fabric, as the asymmetry of the transverse thread floats is during that time still completely present. As abrasion on the running side of the dewatering fabric sets in at the lowest point of the transverse thread floats, the asymmetry of the transverse thread floats becomes less, the longer the dewatering fabric is in use.
It would admittedly be possible to solve the problem of lateral drifting by using a weave with symmetrical transverse floats for the dewatering fabric. With this type of weave there generally exists, however, the drawback that the plane difference between the longitudinal thread floats and the transverse thread floats on the running side are less. Large plane differences are, however, necessary on the running side, since they serve to increase the service life of the dewatering fabric. It is possible when large plane differences are achieved to use thicker transverse threads and to have the transverse thread floats worn down completely by abrasion before the longitudinal threads are exposed to any abrasion.
The asymmetrical transverse thread floats are brought about in that a plurality of longitudinal threads act jointly on one location of the transverse thread float. As consequence, on the one hand, a large plane difference is attained and, on the other hand, the transverse float gets a asymmetrical form, if that location is not disposed in the center of the transverse thread float.
It is known in U.S. Pat. No. 4,592,395 to make the weave to the mirror-image symmetrical in the two halves of the dewatering fabric to the left and to the right of the center line extending in the longitudinal direction, so that the weave diagonal has V-configuration. The difficulty encountered here, however, is that special longitudinal threads must be worked-in in the center of the dewatering fabric in a manner deviating from the remaining weave so as to avoid excessively long transverse thread floats on the running side.